Powder-bed Additive Manufacturing Technique Research Articles

Comparison between laser and TIG welding of electron beam melted Ti6Al4V parts

Abstract A large number of metal parts specific to the aviation, energy, and biomedical industries are produced by the electron beam melting (EBM) method, which is one of the powder bed additive manufacturing techniques. The limited build volume of EBM machines does not allow the production of parts in the desired dimensions. One way to overcome this limitation is to weld small size additive manufactured parts. In this study, EBMed Ti6Al4V tensile specimens were joined by laser (LBW) and tungsten inert gas (TIG) welding. Welding morphologies, microstructures, and mechanical properties of joints were investigated. The main defects in the samples are pore formation and insufficient penetration. The weld zones of TIG samples contain a higher amount of pores than laser samples, and these pores are distributed over the entire area of the weld. The pores are less than 200 µm in diameter. TIG welded samples exhibited higher mechanical properties than laser welded samples. The highest microhardness was measured in the weld zone. Microhardness of laser welded samples are higher than TIG welded samples. While the welding regions of TIG welded samples consist of coarse and acicular α and α + β structures, laser welded samples consist of thin and acicular α′ structure.

Effect of Powder Characteristics on Relative Density and Porosity Formation During Electron Beam Selective Melting of Al2024 Aluminum Alloy

Abstract Defects, such as pores and cracks, can be found in parts fabricated by powder-bed additive manufacturing techniques. The origin of certain defects, such as some voids, can be linked to initial powder quality, which makes it an important factor in the process. Powders used in additive manufacturing processes are produced by different methods such as gas atomization (GA), plasma atomization (PA), and plasma rotating electrode process (PREP); each gives different powder quality. In this study, two different Al2024 powders, produced by electrode induction GA and PREP techniques, were used to investigate the effect of powder characteristics on defect formation during electron beam melting process (EBM). Powders were first characterized by using Hall flowmeter funnel and scanning electron microscope (SEM); then, the EBM process was carried out, and finally, samples were examined by density measurement using Archimedes method, SEM analysis, and tensile test. PREP powder showed higher levels of sphericity and surface smoothness without attached satellites. Consequently, a higher apparent density and decreased flowing time were achieved in PREP powder. Moreover, gas-induced internal pores were observed in GA particles. The results also revealed the average relative density of 96.7% and 99.4% for the parts built by GA and PREP powders, respectively. SEM micrographs confirmed the results of density measurement of the fabricated parts and showed higher degrees of both spherical and irregular-shaped pores in samples built by GA powder. Additionally, they showed deprived mechanical properties due to the higher porosity contents which can form stress-concentrated areas.

Channel design for 3D models with applications in powder-bed additive manufacturing

Purpose For any 3D model with chambers to be fabricated in powder-bed additive manufacturing processes such as SLM and SLS, powders are trapped in the chambers of the finished model. This paper aims to design a shortest network with the least number of outlets for efficiently leaking the trapped powders. Design/methodology/approach This paper proposes a nonlinear objective with linear constraints for solving the channel design problem and a particle swarm optimization algorithm to solve the nonlinear system. Findings Structural optimization for the channel network leads to fairly short channels in the interior of the 3D models and very few outlets on the model surface, which achieves the cleaning of the powders while causing almost the least changes to the model. Originality/value This paper reveals the NP-harness of computing the shortest channel network with the least number of outlets. The proposed approach helps the design of lightweight models using the powder-bed additive manufacturing techniques.

Design and Fabrication of a 3-D Printed Metallic Flexible Joint for Snake-Like Surgical Robot

Snake-like robots have numerous applications in minimally invasive surgery. One important research topic of snake-like robots is the flexible joint mechanism and its actuation. This letter describes the design and fabrication of a new type of flexible joint mechanism that is enabled by metal powder bed additive manufacturing technique. Kinematics and static models of the flexible joint are presented, which can help in designing and controlling the flexible joint. As a compliant mechanism, the fatigue characteristics of the flexible joint is investigated. Finite element analysis (FEA) is performed aiming for optimizing the design process. In the experiment section, model estimation, FEA, and experimental validation are conducted for further understanding the characteristics of the flexible joint. An example design that can survive after 100 000 full loading cycles is demonstrated. Finally, different design variations of the proposed method and a multi-section flexible endoscope using the proposed design are introduced. The proposed flexible joint has the potential not only in reducing the cost of manufacturing and assembling a snake-like surgical robot, but also benefits for developing of more sophisticated three-dimensional snake robotic structure that has an optimized space for embedded sensing and actuation.

Powder-bed additive manufacturing for aerospace application: Techniques, metallic and metal/ceramic composite materials and trends

The current paper is devoted to classification of powder-bed additive manufacturing (PB-AM) techniques and description of specific features, advantages and limitation of different PB-AM techniques in aerospace applications. The common principle of “powder-bed” means that the used feedstock material is a powder, which forms “bed-like” platform of homogeneous layer that is fused according to cross-section of the manufactured object. After that, a new powder layer is distributed with the same thickness and the “printing” process continues. This approach is used in selective laser sintering/melting process, electron beam melting, and binder jetting printing. Additionally, relevant issues related to powder raw materials (metals, ceramics, multi-material composites, etc.) and their impact on the properties of as-manufactured components are discussed. Special attention is paid to discussion on additive manufacturing (AM) of aerospace critical parts made of Titanium alloys, Nickel-based superalloys, metal matrix composites (MMCs), ceramic matrix composites (CMCs) and high entropy alloys. Additional discussion is related to the quality control of the PB-AM materials, and to the prospects of new approaches in material development for PB-AM aiming at aerospace applications.

Surface Treatment of Powder-Bed Fusion Additive Manufactured Metals for Improved Fatigue Life

High-cycle fatigue (HCF) tests were conducted on samples fabricated by two powder-bed additive manufacturing techniques. Samples were tested with as-produced surfaces and after various non-contact surface improvement treatments. Ti-6Al-4V samples were made using both electron beam melting (EBM) and selective laser melting (SLM), while Inconel 625 was fabricated using SLM. Ti-6Al-4V was treated with a commercial chemically accelerated vibratory polishing process, with target material removal of approximately 200 µm from each surface for EBM samples and 100 µm for SLM samples. This technique led to increases in both the number of cycles before failure at a given loading condition and endurance limit (at 107 cycles) compared to samples with as-produced surfaces. The results are interpreted as the reduction in elastic stress concentration factor associated with surface defects where fatigue cracks initiate. SLM 625 was treated with both an abrasive polishing method and laser surface remelting. Both methods led to improvements in surface roughness, but these did not lead to improvements in fatigue properties of SLM 625. For abrasive polished samples, the combination of improved measured surface roughness without fatigue property enhancement suggests that surface material is removed, but the roots of surface defects, where fatigue cracks initiate, were left intact. For laser treatment, the remelted surface layer retained a rapidly solidified microstructure that did not increase the number of cycles before crack initiation even though the surface was smoother compared to the surface prior to polishing.

Structure, Texture and Phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments

3D printing results in anisotropy in the microstructure and mechanical properties. The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy. Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 °C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 °C, 100 MPa, 4 h, furnace cooling). The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction. Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping. Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations. Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments. Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries. Delta-phase formed due to γ″-phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms. The presence of (Nb0.78Ti0.22)C carbide phase, with an fcc structure having a lattice parameter a = 4.43 Å, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis. The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718.

Binder-jet powder-bed additive manufacturing (3D printing) of thick graphene-based electrodes

Additive manufacturing (AM), also known as 3D printing, is emerging as a promising method for the fabrication of complex 3D structures and has the potential to replace the conventional techniques used in the manufacture of commercial devices based on advanced materials. Graphene has shown superior performance in various electronic devices such as electrochemical supercapacitors. However, it remains challenging to produce the thick, high loading graphene-based electrodes required to achieve a high practical energy density in full devices. Herein, we introduce a powder-bed AM technique for the fabrication of crack-free, mm-thick graphene-based electrodes, with high surface area that can be printed in complex shapes. While this technology has the potential to be used in many application areas including energy storage, conversion, and sensing, in this work, we demonstrate their use as high performance supercapacitors. Devices fabricated using thermally exfoliated graphene oxide powder had gravimetric and areal capacitance of ∼260 F g−1 and ∼700 mF cm−2, respectively at 5 mV s−1 in 1 M H2SO4 electrolyte. The supercapacitors retained 80% of their capacity over 1000 cycles. This technique provides a promising route for the fabrication and commercialization of thick, porous graphene-based devices.

A novel additive manufacturing-based technique for developing bio-structures with conformal channels and encapsulated voids

A novel additive manufacturing-based technique for developing bio-structures with conformal channels and encapsulated voids. The network of macro-channels in the structure of cortical bone is crucial, particularly for nutrition. Therefore, a successful bone implant should simulate the real bone architecture by including such channels in its structure. The introduction of additive manufacturing (AM) techniques in the orthopedic implant industry brings the potential of producing customized bone implants that mimic the structure of real bone. However, the depowdering issue in the powder bed AM technique has hindered the creation of macro-sized channels in bone implants composed of biocompatible materials. In this study, we introduce a new method for manufacturing implants which is composed of titanium and includes networks of channels. This technique is primarily based on printing individual components of a sliced structure, followed by depowdering and assembling the components before the sintering process. This new technique has the potential to control the internal features of 3D printed structures. A set of comparative physical and mechanical tests were conducted to characterize the resulting structures. Experimental characterization results showed that the shear strength of the sample that was made by the new technique was reduced by 24%–30%, where the porosity was slightly lower (∼2%) than that of a comparable control sample. However, the new technique had no effect on the compressive strength of the structure.

Study on unsupported overhangs of AlSi10Mg parts processed by Direct Metal Laser Sintering (DMLS)

A well-known limitation of powder bed additive manufacturing techniques for metals is the requirement of support structures on down-facing surfaces. The aim of this research is addressed to the evaluation of geometrical parameters of self-supporting faces and estimation of the achievable tolerance of surfaces built without requiring a support structure. In order to accomplish the purpose of this study a Design of Experiment (DoE) approach is adopted to explore the significance of key parameters such as angle, curvature and slenderness of overhanging surfaces. Deviation between the actual and the nominal CAD geometries is computed, and results are processed following the tolerance unit method defined in international standards (IT grade). At the end of this study, the closest tolerance obtainable on self-supporting faces and related geometrical parameters are identified, limited to Direct Metal Laser Sintering (DMLS) of aluminium parts.

XCT analysis of the influence of melt strategies on defect population in Ti–6Al–4V components manufactured by Selective Electron Beam Melting

Selective Electron Beam Melting (SEBM) is a promising powder bed Additive Manufacturing technique for near-net-shape manufacture of high-value titanium components. However without post-manufacture HIPing the fatigue life of SEBM parts is currently dominated by the presence of porosity. In this study, the size, volume fraction, and spatial distribution of the pores in model samples have been characterised in 3D, using X-ray Computed Tomography, and correlated to the process variables. The average volume fraction of the pores (<0.2%) was measured to be lower than that usually observed in competing processes, such as selective laser melting, but a strong relationship was found with the different beam strategies used to contour, and infill by hatching, a part section. The majority of pores were found to be small spherical gas pores, concentrated in the infill hatched region; this was attributed to the lower energy density and less focused beam used in the infill strategy allowing less opportunity for gas bubbles to escape the melt pool. Overall, increasing the energy density or focus of the beam was found to correlate strongly to a reduction in the level of gas porosity. Rarer irregular shaped pores were mostly located in the contour region and have been attributed to a lack of fusion between powder particles.